add ipython[terminal] dependency group

for gnureadline on OS X and pyreadline on Windows.

These will *not* be installed by default anymore. There will no longer be any checks for dependencies in entry points.

This is a proposal for discussion.

Reasons not to do it, or at least partially:

- many more OS X users actually use the terminal, which is essentially unusable with libedit (Python.org or System Python)
- most people don't understand optional dependency groups in setuptools

supersedes #5440